1. Field of the Invention
The present invention relates to a multipoint focus detecting apparatus which can determine a focus state at each of a plurality of focus detection zones, and which is suitable for an optical instrument such as an SLR camera.
2. Description of the Prior Art
In recent years, SLR cameras provided with a multipoint focus detecting unit for determining a focus state (defocus) at each of a plurality of focus detection zones (focusing points) have been developed. In the optical system of a conventional multipoint focus detecting unit, in addition to the central focus detection zone arranged over the optical axis, one or more off-center focus detection zones are arranged away from the optical axis, and light bundles which form an object image which are passed through the central and off-center focus detection zones are respectively deflected by corresponding mirrors to be incident on corresponding light receiving elements arranged in a horizontal line. For instance, a light bundle of an object image that is passed through the horizontally-elongated central focus detection zone is deflected once by a mirror to be focused on a corresponding horizontally-elongated array of light receiving elements. The image observed through the horizontally-elongated central focus detection zone is divided into two, by a corresponding pair of separator lenses, to be formed as two separate images on the corresponding horizontally-elongated array of light receiving elements. At the same time, another light bundle of the object image which is passed through a vertically elongated off-center focus detection zone is deflected twice by two mirrors to be focused on a corresponding horizontally-elongated array of light receiving elements. The image observed through the vertically-elongated off-center focus detection zone is divided into two by a corresponding pair of separator lenses to be formed as two separate images on the corresponding horizontally-elongated array of light receiving elements. The central and off-center pairs of separator lenses are arranged away from the central and off-center light receiving elements by a predetermined distance.
FIG. 4 is a developed view of an optical path of a conventional multipoint focus detecting apparatus, showing optical paths thereof. Note that none of the aforementioned deflecting mirrors are, for clarity, shown in FIG. 4. A field mask 101, positioned in a plane located at a position optically equivalent with a film surface (i.e., a focal plane of the photographing lens), is provided with two focus detection apertures 101a and 101b which respectively determine two off-center focus detection zones (e.g., a left focus detection zone and a leftmost focus detection zone) arranged away from the center of the field of view. A light bundle La, which is passed through the aperture (e.g., the aforementioned left focus detection zone) 101a and a condenser lens 103a, is firstly deflected by a prism 105a in an outward direction (i.e., in a downward direction as viewed in FIG. 4), and is subsequently deflected by a prism 107a in a direction to be substantially parallel to the optical axis O of a photographing lens (not shown). Subsequently, the light bundle passes through a separator mask 109a provided as an exit-pupil dividing device and a pair of separator lenses 11a to be incident on a corresponding line sensor 113a, so that a couple of object images (a couple of light distributions) are formed on the line sensor 113a. 
On the other hand, a light bundle Lb, which is passed through another aperture (e.g., the aforementioned leftmost focus detection zone) 101b, that is positioned farther from the optical axis O than the aperture 101a, and a condenser lens 103b, is firstly deflected by a prism 105b in an inward direction (i.e., in an upward direction as viewed in FIG. 4), and is subsequently deflected by a prism 107b in a direction to be substantially parallel to the optical axis O. Subsequently, the light bundle passes through a separator mask 109b provided as an exit-pupil dividing device, and a pair of separator lenses 111b to be incident on a corresponding line sensor 113b, so that a couple of object images (a couple of light distributions) are formed on the line sensor 113b. Each of the line sensors 113a and 113b extends in a direction normal to a surface of the drawing of FIG. 4, and the two light bundles which are passed through the two separator masks 109a and 109b are incident on the two line sensors 113a and 113b to be formed as two object images thereon, respectively.
Among the light bundle Lb that is passed through the focus detection aperture 101b, for instance, there is a stray light bundle Lbxe2x80x2 that is incident on the separator mask 109a, rather than the separator mask 109b. If the line sensor 113a receives such a stray light bundle that is passed through the focus detection aperture 101b, the object image of the stray light bundle is formed on the line sensor 113a. This deteriorates the precision in detection of the multipoint focus detecting apparatus. In some cases, the multipoint focus detecting apparatus cannot determine a focus state due to such a stray light bundle.
The present invention has been made in view of the aforementioned problems that reside in conventional multipoint focus detecting apparatuses, and an object of the present invention is to provide a multipoint focus detecting apparatus which can prevent any stray light which may cause a focus detection error from occurring.
To achieve the object mentioned above, according to an aspect of the present invention, a multipoint focus detecting apparatus of a camera is provided, including a plurality of exit-pupil dividing devices for dividing an exit pupil of a photographing lens into a plurality of detection sub-zones, the plurality of detection sub-zones corresponding to a plurality of arrays of light receiving elements arranged in a second direction orthogonal to a first direction in which the exit pupil is divided by each of the plurality of exit-pupil dividing devices; a plurality of pairs of light distribution forming devices, each of the pairs of light distribution forming devices receiving light bundles which are passed through a corresponding pair of the plurality of detection sub-zones to form a corresponding pair of light distributions, respectively, relative positions of which vary in accordance with a variation in position of a focal point of the photographing lens; a focus detection zone determining device that is positioned in a plane located substantially at a position optically equivalent with a focal plane of the photographing lens to determine the plurality of focus detection zones, at least two light bundles which are respectively passed through corresponding at least two of the plurality of focus detection zones being respectively incident on corresponding at least two of the plurality of exit-pupil dividing devices after intersecting each other; and a light intercepting member which is positioned in a vicinity of an intersection between the at least two light bundles so that each of the at least two light bundles is not incident on any of the plurality of arrays of light receiving elements other than a corresponding one of the plurality of arrays of light receiving elements.
Preferably, at least one condenser lens positioned between the focus detection zone determining device and the plurality of exit-pupil dividing devices is included so that the light bundles which are respectively passed through the plurality of focus detection zones pass through the at least one condenser lens.
In an embodiment, each of the plurality of exit-pupil dividing devices includes a separator mask having at least two apertures.
In an embodiment, each pair of the plurality of pairs of light distribution forming devices includes a pair of separator lenses.
Preferably, each of the plurality of arrays of light receiving elements is a CCD line sensor.
Preferably, the focus detection zone determining device is a field mask having a plurality of apertures which determine the plurality of focus detection zones, respectively.
Preferably, the multipoint focus detecting apparatus is incorporated in an SLR camera.
Preferably, the light intercepting member includes an opening.
According to another aspect of the present invention, a multipoint focus detecting apparatus is provided, having a plurality of focus detecting zones arranged in a first pattern on a predetermined focal plane, and a corresponding plurality of arrays of light receiving elements which are arranged in a second pattern that is different from the first pattern. A corresponding plurality of light bundles which are passed through the plurality of focus detecting zones are made incident on the plurality of arrays of light receiving elements, respectively; the plurality of focus detecting zones being off-center with respect to a center of the focal plane. The plurality of focus detecting zones includes a first off-center focus detection zone, and a second off-center focus detection zone which are positioned on a same side on the focal plane with respect to the center thereof, the second off-center focus detection zone being positioned farther from the center of the focal plane than the first off-center focus detection zone. The multipoint focus detecting apparatus includes at least one light deflector which deflects a first light bundle and a second light bundle which are respectively passed through the first off-center focus detection zone and the second off-center focus detection zone in directions to intersect each other; a first light reflector which reflects each of the first and second light bundles deflected by the at least one light deflector; a second light reflector which reflects each of the first and second light bundles, reflected by the first light reflector, towards a corresponding one array of the plurality of arrays of light receiving elements; and a light intercepting member which is positioned in a vicinity of an intersection between the first light bundle and the second light bundles so that each of the first light bundle and the second light bundle is not incident on any of the plurality of arrays of light receiving elements other than the corresponding one of the plurality of arrays of light receiving elements.
Preferably, at least one light deflector is designed and arranged so that the first and second light bundles reflected by the first reflector are incident on the second reflector after intersecting each other, and wherein the light intercepting member is positioned between the first reflector and the second reflector.
Preferably, the multipoint focus detecting apparatus is incorporated in an SLR camera.
Preferably, the light intercepting member includes an opening.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 11-302561 (filed on Oct. 25, 1999) which is expressly incorporated herein by reference in its entirety.